Tài liệu giảng dạy CCNA - module 01 chapter 03 - Data Link Layer Fundamentals

Module 01 InterNetwork OverviewChapter 03 Data Link Layer Fundamentals: Ethernet LANs... Table of Content1 Typical LAN Features for OSI Layer 1 2 Typical LAN Features for OSI Layer 2 3 E

Module 01 InterNetwork Overview

Chapter 03 Data Link Layer Fundamentals: Ethernet LANs

Table of Content

1 Typical LAN Features for OSI Layer 1

2 Typical LAN Features for OSI Layer 2

3 Early Ethernet Standards

4 Recent Ethernet Standards

1 Typical LAN Features for OSI

Layer 1

 The OSI physical layer, or Layer 1, defines the

details of how to move data from one device to

another

 The OSI physical layer defines the standards used

to send and receive bits across a physical

network.

2 Typical LAN Features for OSI

Layer 2

Data Link Function 1: Arbitration

 The specifications for these data-link protocols

define how to arbitrate the use of the physical

medium to avoid collisions, or at least to recover from the collisions when they occur

 Ethernet uses the carrier sense multiple access

with collision detection (CSMA/CD) algorithm for

arbitration

 The CSMA/CD algorithm works like this:

1 A device with a frame to send listens until

the Ethernet is not busy

2 When the Ethernet is not busy, the sender

begins sending the frame

3 The sender listens to make sure that no

collision occurred

4 Once the senders hear the collision, they

each send a jamming signal, to ensure that all stations recognize the collision

5 After the jamming is complete, each sender

randomizes a timer and waits that long

6 When each timer expires, the process starts

over with Step 1

Data Link Function 2: Addressing

 Every computer has a unique way of identifying

itself : MAC address or physical address

 The physical address is located on the Network

Interface Card (NIC)

 MAC addresses have no structure, and are

considered flat address spaces MAC addresses

are sometimes referred to as burned-in

addresses (BIAs) because they are burned into

read-only memory (ROM) and are copied into

random-access memory (RAM) when the NIC

initializes

 0000.0c12.3456 or 00-00-0c-12-34-56

MAC address format

The first six hexadecimal digits, which are administered

by the IEEE, identify the manufacturer or vendor.

The remaining six hexadecimal digits comprise

the interface serial number.

Using MAC addresses

Data Link Function 3: Error Detection

 Most data-link protocols include a frame check

sequence (FCS) or cyclical redundancy check

(CRC) field in the data-link trailer This field

contains a value that is the result of a

mathematical formula applied to the data in the

Data Link Function 4: Identifying the

Encapsulated Data

3 Early Ethernet Standards

10BASE2 and 10BASE5

Standard Topology Medium Maximum

cable length

Transport

coaxial cable

cable

A Repeater

 Standards guarantee minimum bandwidth and

operability by specifying the maximum number of stations per segment, maximum segment length, maximum number of repeaters between stations, etc

 Stations separated by repeaters are within the

same collision domain Stations separated by

bridges or routers are in different collision

domains

10BASE-T Ethernet

 Topology: star (hub)

 Medium: CAT3 UTP.

 Maximum cable length: 100m.

 Transport: half-duplex.

Ethernet 10BASE-T Cabling

Pin 4 Pin 4 Pin 5 Pin 5

Pin 6 Pin 2

Pin 7 Pin 7 Pin 8 Pin 8 Straight-through cable Crossover cable

10BASE-T Hubs

 The hub repeats the signal to all other devices

Reducing Collisions Through LAN

 LAN switches with only one device cabled to

each port of the switch allow the use of

full-duplex operation.

4 Recent Ethernet Standards

Fast Ethernet

 Two of the key features of Fast Ethernet,

as compared to 10-Mbps Ethernet, are

higher bandwidth and autonegotiation.

 Medium: CAT5 UTP, fibre optic.

 Transport: half or full-duplex.

Gigabit Ethernet

 The most important similarity is that the

same Ethernet headers and trailers are

used, regardless of whether it’s 10 Mbps,

100 Mbps, or 1000 Mbps.

 Gigabit Ethernet differs from the slower

Ethernet specifications in how it encodes

the signals onto the cable.

OSI Layer 2 Functions Ethernet Implementation

Arbitration CSMA/CD algorithm.

Addressing 6-bytes-long MAC address.

Error detection FCS in Ethernet trailer

Identifying the type of packet

inside the frame

Protocol Type (2 bytes)—DIX Ethernet

DSAP (1 byte)—IEEE 802.2

SNAP Protocol Type (2 bytes)—

IEEE 802.2 with SNAP header